In this revised application, funding is requested for the continued support of the NIEHS Center for Environmental Health Sciences (CEHS) at the Massachusetts Institute of Technology (MIT). The overall focus of the MIT CEHS is to understand how toxic environmental agents perturb biological systems and to determine how such perturbations may affect human health. Founded in the early 1970' s, the CEHS underwent a renewal in leadership in 2001 and is now Directed by Leona Samson and Peter Dedon. As a result of this change in leadership and in response to reviewer criticisms, the Center has been completely reorganized. The current CEHS membership consists of 32 faculty members derived from a total of 8 MIT departments (in the Schools of Science and Engineering), plus three Departments in the Harvard School of Public Health (HSPH) and one in the Harvard Medical School (HMS). The MIT departments are Biology, Chemistry, Biological Engineering, Chemical Engineering, Civil & Environmental Engineering, Mechanical Engineering, Nuclear Engineering, Electrical Engineering & Computer Science; the HSPH departments are Epidemiology, Nutrition and Environmental Health. The current Research Cores, each led by an MIT Professor, are (i) the Mutation and Cancer Research Core (led by Peter Dedon); (ii) the Bioengineering for Toxicology Research Core (led by Linda Griffith); and (iii) the Environmental Systems and Health Research Core (led by David Schauer). The current Facilities Cores are (i) the Bioanalytical Core (led by John Wishnok), (ii) the Genomics and Bioinformatics Core (led by Peter Sorger) and (iii) the Animal Models and Pathology Core (led by James Fox). The services available through the three Facilities Cores include sophisticated mass spectrometry, accelerator mass spectrometry, chromatography, transcriptional profiling and computational analysis of such data, transgenic and knock out animal production, pathology services, and state-of the-art microscopy and imaging. Other units include the Administrative Core, the Pilot Project Program and the Community Outreach and Education Program (COEP). The Center also co-sponsors three departmental seminar series, and sponsors a CEHS member seminar series, an annual retreat and several other mechanisms designed to nurture interactions among CEHS members and to promote the activities of the CEHS and an awareness of environmental health science in the MIT community.